Fatigue strength improvement of a 4140 steel by gas nitriding: Influence of notch severity

Abstract

Fatigue failure of a gas-nitrided 4140 steel under axial cyclic loading results from a competition between surface crack initiation in the nitrided case and internal "fish-eye" cracking inside the core material. When nitriding is deep enough, the internal mechanism prevails in smooth specimens and fatigue strength improvement as compared to base metal is about 20%. In the present study, three V-notched specimens (blunt -medium -severe) are designed to be representative of the stress gradient (i) in a small rotary bending specimen, (ii) at the root of a gear tooth, and (iii) at the root of a very sharp notch. The cracking mechanism depends on the notch severity. The nitrided blunt notch fails from a fish-eye nucleated at the case/core boundary whereas the medium and sharp notches fail from surface cracks. The high-cycle fatigue strength improvement varies from 80% for the blunt notch and to more than 100% for the sharp notch. The notch fatigue behaviour of nitrided steel is discussed by comparing the evolutions of internal and surface fatigue strengths with relative stress gradient.

Study of metadynamic recrystallization behaviors in a low alloy steel

“…Due to its good balance of strength, toughness and wear resistance, 42CrMo high-strength steel is widely used for many general purpose parts including automotive crankshaft, rams, spindles, etc. In the past, many investigations have been carried out on the behaviors of 42CrMo steel (Limodin and Verreman, 2006). Smoljan (2004) found that it is possible in effective way to improve a steel strength and toughness by the combined cyclic heat treatment consisted of temperature cycling followed by twice quenching.…”

Propriedades do aço AISI 4140 nitretado a gás

“…O aumento do parâmetro de rede é proveniente da presença de nitrogênio intersticial e, após a saturação do elemento, pela formação de nitretos e que geram um aumento de tensão residual compressiva na camada nitretada. Esse aumento no nível de tensões residuais na superfície de um componente mecânico melhora a sua vida em fadiga [6] [14].…”

Low Cycle Fatigue Behaviour of Nitrided Layer of 42CrMo4 Steel

“…The interest of this treatment in the industrial applications justifies the number of studies which were devoted to this subject [1][2][3][4][5][6][7][8][9]. This reveals that the in-depth penetration crack resistance of hardening elements (N 2 ), of the hardness of these layers [4,5,6,8] as well as the distribution of the residual stresses [7,9,10].…”